Depending on the model, the X-flow valves are basically US Cleveland 351 2V rip offs, made in a smaller head diameter, and very slighly longer in overall length:
Intake Diameter. 1976 to 1984, 1.74" diameter or 1985 to 1993, 1.80". Note: 1.72" valves are often used if the orignals are re-cut. There is sufficient valve seat for this, and the total valve height is not altered much
Intake guide. 1976 to 1993, 11/32" nominal, or 8.66 mm blueprinted. In inches, blue print dimension is 0.3409"
Intake length (total). 1976 to 1993, 5.307" nominal, or 134.80 mm blueprinted. The valve has a 125 thou recess in the valve head.
Exhast Diameter. 1976 to 1993, 1.51" diameter. Note: less than 1.51" valves are often used if the orignals are re-cut. There is sufficient valve seat for this, and the total valve height is not altered much
Exhast guide. 1976 to 1993, 11/32" nominal, or 8.66 mm blueprinted. In inches, blue print dimension is 0.3409"
Exhast length (total). 1976 to 1993,5.052" nominal, or 128.32 mm blueprinted. The valve has a 125 thou tulip section in the base.
In terms of options, the Cleveland valves are exactly the same, but have about 77 thou lower total lengths on the intake valves. It is not worth redoing a bunch of cheap 351C valves to fit your X-flow unless you install lash caps and get the machining done cheaply. One guy here has installed 351 2v stainless intakes, turned down from 2.047" to 1.96".
Over here, some guys have used some American made L34 Holden exhast valves, as they are the same length and guide size as the X-flow exhast valves, but are 100 thou bigger.
Funny thing is, all the aftermarket valves are often ex USA anyway. Segedins in the North Island import white box US Performance valves through there US supliers at very competitive rates. Becasue of the shear production capacity, a good deal of aftermarket x-flow gear, such as cams, timing chain sets and valves are US made.
